In screw compressors the bores for the rotors overlap. The overlapping bores create cusps in the nature of the waist of a figure eight. One of the cusps in the normal lactation for one form of a mechanical unloader which forms a portion of the bore and coacts with the rotors as it moves axially in the cusp to unload and to control the V1 or discharge volume to suction volume ratio, of the compressor. The unloader is normally driven by a solenoid. To provide a greater degree of control, it is common to provide a variable speed drive which controls the motor by changing the frequency of the electric power being supplied to the motor by the variable speed drive.